A power supply is an electronic device that is widely used in daily life. Generally, an electric appliance has a power supply for providing electric power to internal electronic components of the electric appliance. For example, an AC/DC power supply usually comprises a power factor correction (PFC) circuit and an LLC resonant converter. After an AC voltage (e.g., from a utility power source) is received by the PFC circuit and the power factor of the AC voltage is corrected by the PFC circuit, a transition DC voltage is outputted. The LLC resonant converter is used for resonantly converting the transition DC voltage into an output DC voltage with a desired voltage level.
However, during operation of the AC/DC power supply, the received AC voltage has a low frequency (e.g., the 90-120 Hz ripple). Consequently, the transition DC voltage from the PFC circuit and the output DC voltage from the LLC resonant converter also contain low frequency ripples. These low frequency ripples may result in additional power loss, cause abnormal operation of the AC/DC power supply, or even generate noise and undesired sound.
Conventionally, the AC/DC power supply with the LLC resonant converter uses an analog filtering approach for inhibiting the low frequency ripple. That is, an analog filtering circuit such as an RC filtering circuit or an LC filtering circuit is connected to an output terminal of the LLC resonant converter. The analog filtering circuit is used for filtering off the low frequency ripple that is contained in the output DC voltage from the LLC resonant converter. After the resistance, capacitance and/or inductance of associated components of the AC/DC power supply are calculated and specially designed, the analog filtering circuit can effectively inhibit the low frequency ripple to a certain extent. However, the use of the analog filtering circuit increases the fabricating cost of the AC/DC power supply and occupies the layout space of the AC/DC power supply.
For solving the problem of using the analog filtering circuit, a conventional method is directed to increase the gain value of the LLC input voltage at the low frequency in order to reduce the low frequency ripple of the AC/DC power supply output voltage. In accordance with this method, the LLC output voltage and the gain value of a gain filter are multiplied to generate a gain signal and increase the gain value of the LLC output voltage at the low frequency, and the LLC input voltage is compensated by the gain signal to increase the gain value of the LLC input voltage at the low frequency. FIG. 1A is a plot illustrating the relationship between the gain value of the LLC input voltage and the frequency after compensated by the gain signal. FIG. 1B is a plot illustrating the relationship between the phase of the LLC input voltage and the frequency after compensated by the gain signal. However, since the LLC output voltage and the gain value of a gain filter are multiplied to increase the gain value of the LLC input voltage at the low frequency, the zero and pole locations are changed. If the LLC input voltage is directly compensated by the gain signal with the changed zero and pole locations, a significant phase impact occurs. Please refer to the circumscribed zone A of FIG. 1A. After the LLC input voltage is compensated by the gain signal, the gain value of the LLC input voltage at the frequency of about 100 Hz is increased. However, the circumscribed zone A′ as shown in FIG. 1B indicates that the increase of the gain value results in a phase impact of about 90 degrees to about −90 degrees. Because of the significant phase impact, the AC/DC power supply is unstable.
Therefore, there is a need of providing a ripple compensation circuit of a power supply and a compensating method thereof in order to overcome the above drawbacks.